It has been widely practiced to vaporize an active ingredient such as insecticide, perfume, etc. by heating a composition which contains the same and which is adsorbed on a mat, fiber plate or like porous substrate with the use of a heating device such as an electric mosquito-destroying device. However, this method involves the use of a mat impregnated with an inevitably limited amount of the active ingredient and requires replacement and disposal of the mat after use. The method also has the serious drawback of the the fugacity rate of active ingredient being reduced with the lapse of time. Further, the method is disadvantageous in that an effective volatilization ratio is as low as about 50%, permitting about 10% of the active ingredient to remain in the substrate and failing to retain a stable fumigating effect over a prolonged period of time. For these reasons, the method entails great economical disadvantages.
In order to overcome the problems of replacing waste mats and the loss of the fumigating effect in a short period of time, an improved method capable of maintaining the desired effect for a prolonged period of time has been proposed. The improved method involves the thermal vaporization of a solution of active ingredient being drawn up by an absorbing body. The proposal provides a device comprising a container for a solvent solution of active ingredient, an absorbing body of felt or like material for absorbing the solution and a heating means for heating the absorbing body so that the solution is volatilized off the top of the absorbing body (a device of this type will be referred to as "absorbing-vaporization device" throughout the specification and claims). However, when any of the absorbing-vaporization devices is operated in practice, the heating of the absorbing body causes the solvent in the solution to preferentially vaporize, thereby gradually concentrating the solution within the absorbing body. This is likely to form resinous solids in the body and to carbonize the same. Such phenomena give rise to clogging in the absorbing body, whereby subsequent absorption and volatilization of the solution are inhibited. This inhibition results in failure to retain the desired fumigating effect over a prolonged period of time, decrease in the desired effect with the passage of time, and reduction in the effective volatilization ratio, with a significant portion of the active ingredient being left unused. The causes for these problems attendant on the foregoing method may be attributed to various factors. These factors include the selection of an absorbing body, solvent and active ingredient, the concentration of the active ingredient, heating conditions, etc. The problems have been difficult to solve.